This invention relates to the measurement of process variables by sensors, and particularly to the processing of sensor signals to obtain an accurate weighted average based on sensor signals provided by three or more sensors which are measuring the same process variable.
There are, in existence, many sensing systems for measuring a variety of variables including, for example, temperature, pressure, level, flow rate, amplitude, voltage, current, power, etc. In those circumstances where it is particularly critical that the measured value be accurate and protected against failure, it is common practice to employ three redundant sensors to measure the same variable. This is often referred to as triple redundancy.
One environment in which triple redundancy is employed is a nuclear power plant. In a nuclear power plant, certain process variables are measured by three redundant sensors in order to ensure the continuous availability of an accurate sensing signal, without any down time due to a failure of the sensor itself. The reliability of such systems employing triple redundancy is significantly enhanced if the accuracy of the final numerical value which is obtained can be maintained even if one of the three sensors fails to operate. Failure of a sensor typically occurs in one of three modes with approximately equal probability. The first mode is a failure with a zero output, the second mode is a failure with a very high output, and the third mode is a failure in such a way that a value is produced which drifts (in finite time) away from the correct value, due to a component or material failure.
Prior art methods and apparatus have applied consistency tests to the three sensed values, and as soon as one of the three values fails a consistency test, that value is removed from any influence on the final numerical value. For example, the inconsistent value may be immediately removed from an averaging calculation. The discontinuous nature of this abrupt removal of the inconsistent value can produce steps in the output value, which may in turn produce deleterious effects in downstream operations. Further, oscillations may be generated when a given signal is on the verge of a change of state from one set of averages to another at the time an inconsistent value is removed from the averaging calculation. Thus, there is a need in the art for a method and apparatus for redundant measurement of variables, which produces a continuous output and which takes into account the fact that transients may occur in the system being monitored.